The ground war over gun control after the Newtown, Conn. massacre has begun.

Across the country, members of Congress are being hit with grassroots campaigns and ads aimed at pushing them to one side or the other in the current debate over gun violence.

On Friday, the remnants of President Obama’s reelection campaign will hold its first “Day Of Action” since the November election, rallying its millions of supporters to join the call for gun control legislation at grassroots events nationwide.

The group, Organizing For Action, is also putting ads online that will “target more than a dozen members of Congress” who haven’t signed onto Obama’s gun control agenda, according to USA Today.

OFA’s move caps off a week that has seen gun control tested at the ground level of politics, where the races are won and where the mountains are moved. Thus far, most of the post-Newtown gun control debate has taken place at presidential press events and Senate committee hearings. Now it’s part of actual politicking.

The National Rifle Association is targeting several members with ads of its own, running full-page newspaper spots in Arkansas, Louisiana, Maine, North Carolina and West Virginia.

In the past, this kind of saber-rattling by the NRA would likely scare moderate Democrats into submission on issues like universal background checks for firearms purchases, which polling shows the public supports but the NRA opposes.

This time around, though, gun control advocates and their allies say the ground fight favors them. “I believe the price to be paid politically will be to those who refuse to act, who refuse to step forward [on gun violence],” Vice President Biden said Thursday. “Because America has changed on this issue.”

But it’s not just a shifting public attitude gun control advocates are relying on. They’ve also got wealthy allies in the field they’ve never had before, giving them resources they’ve never had. New York City Mayor Mike Bloomberg’s war on the NRA put him at the center of the Illinois special election to replace Rep. Jesse Jackson Jr (D).

Bloomberg’s PAC spent heavily to back candidate Robin Kelly (who’s now expected to win) and clear the field for her with ads highlighting NRA-friendly comments made by her competitors.

Bloomberg has made it clear he intends to do more of of this, and he’s got powerful allies in former Rep. Gabby Giffords (D-AZ) and her husband, Mark Kelly, who have also pledged to get involved in races on behalf of stronger gun laws.

The NRA still has a lot of sway in Washington and it still has a lot of money to fight political battles in the field. But there are signs it’s losing the DC fight — a bipartisan group of senators is reportedly close to a deal on universal background checks.

That’s a big victory for gun control advocates. (The House is a different story, though some conservatives have expressed an openness to expanded background checks.)

Having pushed the conversation in its direction in DC, the gun control crowd has now turned its attention to the ground, where it will test the NRA’s grassroots strength against the nascent political machinery of the new gun control coalition.